Idea: adding Zmij for faster float-to-string parsing?

TBH, what we really need is someone to figure out a language feature for things like the allocator shim so that it can be less special and we can use that mechanism for more things -- like this.

Then core can offer a default implementation that's a fine middle-ground, but people can plug in different things in their binaries depending on their needs: size-optimized ones if you only do this for a couple parameters in a CLI, one that calls out to the C library for embedded that has a C version already, a big-but-fast one for something doing a ton of floats in JSON, etc.